This description relates to printed circuit transformers.
A DC-to-DC transformer apparatus, called a Sine Amplitude Converter (“SAC”), is described in U.S. patent application Ser. No. 10/066,418, filed Jan. 31, 2002, and U.S. patent application Ser. No. 10/264,327, filed Oct. 1, 2002 (the “Factorized Applications”, incorporated in their entirety by reference). One embodiment of a SAC, shown in FIG. 1, comprises a series-resonant full-bridge converter operated at a fixed frequency slightly below a characteristic resonant frequency of the converter to provide for switching at times of zero-voltage and zero-current.
Some embodiments of the SAC operate in a “low-Q” configuration, where the “quality factor,” Q, of a series resonant converter operating at resonance is defined in the Factorized Applications, as Q=ZL/Req, where ZL=2π*fR*LR is the total inductive impedance of the resonant circuit at the resonant frequency, fR; where the inductance LR includes all discrete, leakage and circuit parasitic inductances, reflected into the transformer primary and in series with the resonant circuit; and where Req is the total equivalent series resistance of the circuit, reflected to the transformer primary and including, resistances of windings, ON-state resistances of switches, rectifiers and resonant capacitors.
A SAC uses principles of resonant charge transfer so the quality factor, Q, does not directly reflect cycle-by-cycle losses in a SAC. Rather, the losses in the resonant tank of a SAC are proportional to the equivalent series resistance Req. In some SACs, the equivalent series resistance Req is minimized and the transformer is designed to minimize leakage inductance. Use of a low-Q resonant circuit generally provides higher bandwidth and shorter transient response time, together with greater inherent stability.
Herbert, U.S. Pat. No. 4,665,357, “Flat Matrix Transformer” describes a “matrix transformer” that has interdependent magnetic circuits, arranged in a matrix, between and among which electrical conductors are interwired, the circuits and conductors cooperating to behave as a transformer. Matrix transformers generally have magnetically decoupled magnetic cores, each having one or more coupled windings, the windings on different cores being interconnected to form the transformer structure. See, for example, Hibbits, U.S. Pat. No. 3,323,091, “Multicore Transformer Including Integral Mounting Assembly”; in Papaleonidas, U.S. Pat. No. 3,477,016, “Transformer System Including a Large Number of Magnetically Independent Transformer Elements”; and in three papers by Ngo, Alpizar and Watson: “Development and Characterization of a Low-Profile Matrix Transformer”, HFPC May 1990 Proceedings; “Modeling of Magnetizing Inductance and Leakage Inductance in a Matrix Transformer”, IEEE Transactions on Power Electronics, Vol. 8, No. 2, April 1993; “Modeling of Losses in a Sandwiched-Winding Matrix Transformer”, IEEE Transactions on Power Electronics, Vol. 10, No. 4, July 1995.
FIG. 2 shows an exploded perspective view of a portion of a matrix transformer having a serpentine winding that includes a pair of conductive patterns 10a, 10b (e.g., copper etch) on surfaces of one or more substrates 12a, 12b (e.g., printed circuit boards (“PCBs”)). Current enters conductive pattern 10b at the location marked “A” and exits conductive pattern 10a at the location marked “B.” The conductive patterns 10a, 10b are connected in series by, e.g., conductive interconnections, not shown. The patterns carry a current in the direction indicated in the Figure by the arrows. Magnetic core pieces 16a–16c, 17a–17c pass through holes 14a–14f, 15a–15f in the substrates 12a, 12b so that opposing pairs of core pieces (e.g., core pieces 16a and 17a) form an essentially closed permeable flux path. The term “flux path”, as used herein, refers to the principal path followed by the flux that links a pair of transformer windings, as distinguished from minor flux paths, such as those associated with, e.g., leakage flux or fringing fields). In the apparatus of FIG. 2, each flux path formed by each core pair is coupled by two turns formed by the conductive patterns 10a, 10b and each core pair creates a magnetic flux path that is essentially independent of the magnetic flux path formed by the other core pairs.
Matrix transformers comprising serpentine windings are described in the three papers by Ngo, Alpizar and Watson, ibid; in Leno, U.S. Pat. No. 2,943,966, “Printed Electrical Circuits”; in Yerman et al, U.S. Pat. No. 4,959,630, “High-Frequency Transformer” and U.S. Pat. No. 5,017,902, “Conductive Film Magnetic Components”; and in Roshen, U.S. Pat. No. 5,381,124, “Multi-Turn Z-Foldable Secondary Winding for a Low-Profile Conductive Film Transformer.”
Walters, U.S. Pat. No. 5,300,911, “Monolithic Magnetic Device With Printed-Circuit Interconnections” describes a monolithic magnetic device having transformer elements having single turn primaries and single turn secondaries fabricated on a plate of ferrite which has the outline of a ceramic leadless chip carrier. Each of the magnetic elements has a primary winding formed from a copper via plated on the ferrite. Each element's secondary is another copper via plated over an insulating layer formed over the first layer of copper. The elements' primaries are interconnected on the first copper layer and the elements' secondaries are interconnected on the second copper layer. The configuration and turns ratio of the transformer are determined by the series and or parallel interconnections of the primary and secondaries.